This invention relates to a process for preparing surface-treated steel strips adapted for electric resistance welding, and more particularly, to a process for preparing surface-treated steel strips having such improved weldability as to permit can bodies to be joined into food cans by electric resistance welding as well as improved corrosion resistance after lacquer coating.
Among food can-forming materials there have been most widely used tin-coated steel strips generally called tin plates. In order to join the mating edges of a can body, conventional soldering techniques were previously used. Because of the toxicity of lead contained in conventional solder, pure tin solder has recently become prevalent. The pure tin solder, however, has a technical problem in making a joint because of inferior wettability during the soldering process and is so expensive as to create the economic problem of increased manufacture cost.
On the other hand, in recent years, food containers have enjoyed the development of inexpensive, competitive materials such as polyethylene, aluminum, glass, processed paper and the like. Despite their significantly improved corrosion resistance among other advantages, tin plate cans having expensive tin thickly coated thereon to a coating weight of as great as 2.8 to 11.2 g/m.sup.2 require a relatively high cost of manufacture and have encountered severe competition.
In order to overcome the above-described drawbacks of tinplate cans, electric resistance welding of can bodies has recently replaced the conventional soldering technique and become widespread. There is the need for can-forming steel compatible with electric resistance welding.
In addition to tinplate discussed above, tin-free steel of chromium type is another typical example of conventional can-forming steel. The tin-free steel is prepared by carrying out an electrolytic chromate treatment on steel to form a layer of metallic chromium and hydrated chromium oxides on the surface. Since the relatively thick hydrated chromium oxide film on the surface has a relatively high electric resistance, the chromated steel is ineffectively welded to form a weld of insufficient strength and thus unsuitable as welded can-forming steel despite its economic advantage.
Since other can-forming materials are also inadequate as welded can-forming material, a variety of proposals have been made. One example is nickel-plated steel, typically "Nickel-Lite" announced by National Steel Corporation of the U.S. which is prepared by plating a steel strip with nickel to a thickness of about 0.5 g/m.sup.2 followed by a conventional chromate treatment. Inferior adhesion of lacquer has limited the spread of this nickel-plated steel.
Another example is "Tin Alloy" announced by Jones & Laughlin Steel Corporation of U.S. This is prepared by thinly coating a steel strip with tin to a thickness of about 0.6 g/m.sup.2 and effecting tin fusion or reflow followed by a conventional chromate treatment. Unfortunately, rust resistance and lacquer adhesion are insufficient.
In general, can-forming steel sheets intended for electric resistance welding are required to exhibit improved weldability and corrosion resistance after lacquering. These requirements will be explained in detail. There must be a proper welding electric current range within which a weld zone having sufficient weld strength is provided at the end of welding without any weld defects such as so-called "splashes". Since welded cans are filled with foodstuffs after lacquer coating, the underlying steel must have sufficient adhesion to lacquer to take full advantage of the corrosion prevention of the lacquer film. Furthermore, despite defects unavoidably occurring in a lacquer film, the improved corrosion resistance of the underlying steel itself prevents corrosion from proceeding.
The inventors previously proposed in Japanese Patent Application Nos. 58-124235 and 58-222372 a process for preparing a thinly tin-coated steel strip capable of satisfying both weldability and corrosion resistance after lacquering. These applications are directed to a process for preparing a surface-treated steel strip adapted for electric resistance welding, comprising the steps of forming a first layer of iron-nickel alloy on a steel strip, the first layer having a weight ratio of Ni/(Fe+Ni) in the range between 0.02 and 0.50 and a thickness of 10 to 5,000 angstroms; forming a second layer of tin or iron-tin-nickel alloy on the first layer by tin plating to a coating weight of 0.1 to 1 g/m.sup.2 of tin and optionally, causing the tin to reflow; and forming a third layer on the second layer by effecting an electrolytic chromate treatment. The third layer consists essentially of metallic chromium and hydrated chromium oxide in a total amount of 5 to 20 mg/m.sup.2 calculated as elemental chromium. The electrolytic chromate treatment is controlled such that the following relationships: EQU 2.ltoreq.X and EQU 5.ltoreq.X+Y.ltoreq.20
are met provided that X represents the amount of metallic chromium in the third layer and Y represents the amount of hydrated chromium oxide in the third layer calculated as elemental chromium, both expressed in mg per square meter. These processes are successful to a substantial extent in producing thinly tinned steel strips having improved weldability and corrosion resistance.